Fibre optic cables are widely used in optical instruments. They are practical means for guiding light for illumination and light to be measured. Fibre optic cables usually consist of a large number of thin fibres, to make the cable flexible. Each fibre usually has a cladding which is made of material with a lower refractive index than the core material of a fibre. The cladding enables total reflection at the interface between the core and the cladding of the fibre. The number of fibres and the diameter of fibres depend on application. The number of fibres is often in the range of 50-500, and the diameter of each fibre is often in the range of 50-600 μm, but numbers and diameters of fibres beyond these ranges are also quite possible. In many applications it is important to achieve small transmission loss of light. When light is applied at an end of a fibre optic cable, only part of the light enters the fibres. One reason for the losses in the optical interface is that part of the applied light enters the spaces between adjacent fibres at the surface of the cable end. This space between the individual fibres can be reduced by fusing the ends of the fibre optic cable. The fusing is usually done by heating and pressing the bundle of fibres. Initially the fibres have circular cross sections, which become hexagonal when the fibre bundle has been fused. In order to avoid interspaces between the fibres, it is useful to avoid using attachment glue or cement in fusing.
In the fusing it is necessary to apply a pressure on the fibre bundle in order to remove the hollow spaces between fibres and to achieve a cable end with small loss of light intensity. It is useful to use a constant pressure to each point of the transversal outline of the bundle. When the bundle has a circular outline, the bundle will remain its form during the fusing, which leads to a homogeneous optical interface with no aberration in the arrangement of the fibre ends. There are several methods for applying a constant pressure to circular fibre bundles of varying sizes. However, providing a fused bundle end with some other than circular outline would require special tools, which are designed for a specific bundle which has a determined diameter and determined number of fibres with determined diameters and materials. Also, achieving high homogeneity and other optical characteristics for a fused, non-circular bundle would be difficult if not impossible. Fusing can also be made by applying cohesive agent in between the fibres, but then the aim of removing the interspaces between the fibres is not at all achieved.
In many applications where optic cables are used, the object to be illuminated is not circular but has some other form, such as rectangular. Also, when light is received into an optic cable, the light source/emission source may have a non-circular form. Such situations are often encountered e.g. is the field of optical measurement equipment used in clinical laboratories. Basic operating principles of optical measurement of clinical samples are next briefly described.
The routine work and also the research work in analytical biochemical laboratories and in clinical laboratories is often based on different tags or labels coupled on macromolecules under inspection. The typical labels used are different radioactive isotopes, enzymes, different fluorescent molecules and e.g. fluorescent chelates of rare earth metals.
The detection of labels can be performed by utilizing its natural biochemical function, i.e. to alter the physical properties of molecules. In immunoassays colourless substances are catalysed into colourful substances or non-fluorescent substances are catalysed into fluorescent substances.
The colourful substances are measured with absorption, i.e. photometric measurement. In the photometric measurement the intensity of filtered and stabilized beam is first measured without any sample and then a sample inside a well of a sample plate is measured. The absorbance i.e. the absorption values are then calculated.
The fluorescent measurement is generally used for measuring quantities of fluorescent label substance in a sample. The most photoluminescence labels are based on molecular photoluminescence process. In this process optical radiation is absorbed by the ground state of a molecule. Due to the absorption of energy the quantum molecule rises into higher excited state. After the fast vibrational relaxation the molecule returns back to its ground state and the excess energy is released as a photon.
A further commonly used measurement method is chemiluminescence measurement where emission is due to a chemical reaction, and emission of a substance is measured from a sample without excitation by illumination. Thus a photoluminometer can also be used as a chemiluminometer.
The typical instruments in analytical chemical research laboratories are the different spectroscopic instruments. Many of them are utilizing ultraviolet (UV) visible or near infrared (NIR) region of electromagnetic spectrum. The instruments are most often multi-label plate readers, but they may also be may be spectrophotometers or spectrofluorometers. Such instruments may have one or two wavelength dispersion devices, such as monochromators. With a controllable dispersion device it is possible to perform the measurements throughout the required optical spectrum. To achieve efficient and accurate measurements of small samples, it is often necessary to maximize the intensity of excitation light at the sample, and to maximize the optical acquisition efficiency of the emission signal received from the sample.
A monochromator generally has a dispersive component and an input aperture and an output aperture for input and output light beams respectively. The output aperture also serves to select the light beam of determined wavelength from the light spectrum. An end of an optical cable can be coupled to an input aperture at a monochromator side wall. An end of another optical cable can be coupled to an output aperture at a monochromator side wall. The dispersive component spreads the light in different angles as a function of the wavelength of light, and therefore the transmittance band as a function of wavelength has round slopes if the monochromator has circular apertures. However, a narrow transmittance band with steep slopes would usually be desirable. In order to achieve a narrow transmittance band the aperture should be narrow. Therefore it is useful to use a slit as an aperture, wherein the slit is rectangular with the width smaller than the height. In order to reduce the attenuation of the light beam, it is also useful to have an input aperture of the same form as the output aperture.
A monochromator can potentially have a narrow transmittance band with steep slopes, and it is also possible to have stepless adjustment of the band pass wavelength, which is not possible with optical filters. However, due to the interface problems the transmission efficiency of the monochromator may low and the transmittance band as a function of wavelength is wide and has round slopes. Therefore optical filters are often used instead of monochromators in optical instruments.
Optical measurement instruments may also have other locations where a circular cross section of the transferred light beam is not suitable. For example, sample wells may have a rectangular shape, and light sources, such as flash lamps, may have a non-circular light emitting area.
There are also several other applications than optical measurement instruments where a circular cross section of a light beam is not suitable. For example, efficient lasers are used in industrial machining and welding purposes. The laser beam is generally guided from the laser source to the object via fibre optic cables. The light beam received from the end of the optic cable is directed to the surface of an object, wherein the beam has circular cross section. When the laser beam is moved along the object, the treated area forms a line on the object. When the laser beam has a circular cross section, less energy is applied to the edges of the treated line than to the centre of the treated line. However, it is often required to treat the object homogeneously within the whole treated area. For example, this is the case when removing oxides from metal surfaces with laser. Such a homogeneous treatment is not possible or it is complicated with a laser beam which has a circular cross section.